I always make the point to my students that Sherlock Holmes has become more than just a world- famous literary character. He's also an iconic image. There aren't many characters who could be recognized by their silhouettes alone. I can think of only three, really: Mickey Mouse, Snoopy, and Sherlock!
Because Sherlock's shadow is so easy to recognize, he's become a staple of advertising. Slap him on a sign and people will know that you're selling something interesting! You might use him as a link to a product that is mentioned in the canon (pipes, hats, magnifying glasses) or simply to indicate your store is filled with unique 'finds' that might appeal to the curious.
Here are two examples I've spotted during my travels. The first is from a pipe and cigar store in Prague. It was actually the first interesting thing I saw on the street when we left our hotel. To me, that signalled we were going to have a wonderful trip!
And just this week, while on a quick Spring Break trip to the Carolina low country, I spotted this sign in the town of Beaufort. It marked a little knick-knack shop that is probably popular with tourists. Would Sherlock approve of the store's name? Hmmm...well, if I read "The Blue Carbuncle" correctly, he didn't immediately give back that precious stone. Maybe he hung to it---if so, this title certainly fits!
So where's the most interesting or unusual place you've seen Sherlock on a sign?